Notices, from the Minute Books of the Geological Society. 569 



surface exhibits a plane composed of one description of deposit. By detaching the 

 moveable part, a valley of denudation is exposed, and in consequence of a sUght 

 inclination being given to the valley, the difference in the stratification which a 

 series of horizontal beds may exhibit at the extremities of a valley is clearly 

 shown. 



2. Number 2. represents a portion of the Newcastle coal-field. The top exhibits 

 an undulating surface, and the outcrop of a coal-seam, once apparently coextensive 

 with the whole area of the district, but now limited, owing to denudation, to its 

 highest part. In the lowest portion of one of the valleys, the outcrop of another 

 seam is exposed for a short distance. The mode of working a bed of coal is 

 shown by removing the upper portion of the model. 



3. Number 3. exhibits one side of a valley of denudation. The strata appear to 

 be horizontal, and in their original relative position, but by referring to the sides 

 of the model a certain amount of displacement is shown ; and the effects of a still 

 greater dislocation may be exhibited by removing the under portion of the fore-part 

 of the model, and placing in the position it occupied, the piece which rested on it : 

 by withdrawing the other upper portion, one side of a valley of denudation is again 

 displayed, and though the amount of dislocation is great, only a simple alternation 

 of similar beds is visible on the surface. 



4. The next model illustrates the errors which might be committed in estimating 

 the number of coal-seams in a district by surface indications. Along the two in- 

 clined ridges there are apparently five beds of coal alternating regularly with the 

 usual measures ; and it is only by endeavouring to follow them across the valley of 

 denudation, and laterally beyond the ridges, that the true characters of the district 

 can be discovered. The seams are then found to terminate abruptly, and the 

 observer would infer first, that the coal-measures are affected by faults ; and 

 afterwards, from a series of careful researches, aided by borings, that there are in 

 reality but three distinct seams, — the two which crop out around the culminating 

 ridge, and another, which underlies, and is visible only in the lower portions of 

 the valley of denudation, where it is twice exposed. The great amount of dis- 

 location thus ascertained would lead the practical miner to abandon a district ap- 

 parently of great value. An oblique intersection of the model illustrates the com- 

 plications produced in the interior of such a district, the vertical sides show the 

 number of faults as well as the amount of dislocation attending each ; and the true 

 stratigraphical structure is exhibited by the back. 



5. In the last model the denudation is at right angles to the direction of the 

 strata, and consequently exposes the apparent vertical succession of beds ; but in this 

 it is parallel to their direction, and therefore has laid bare only a limited amount of 

 strata. No coal-seams are visible, nor any indications of the subjacent mineral 



